Carbon fibers are high value products with a rapidly growing range of applications. Precursor materials used for carbon fiber manufacture include primarily polyacrylonitrile (PAN) and Pitch. Since both of these materials originate from the petrochemical industry, raw material costs have been increasing, and there is interest in finding precursor materials which are not directly coupled to the price of oil.
Lignin has been suggested as a promising lower cost precursor material for carbon fiber manufacture. Lignin is the most abundant organic material on earth after cellulose, and makes up about one quarter to one third of the mass of dry wood. It is the major by-product of the pulp and paper industry and is separated from the cellulose using pulping processes. During these processes the lignin is solubilized by cooking chemicals and migrates from the wood chip to the cooking liquor. At the end of the pulp cook the spent cooking liquor with its load of organic material including lignin, now called black liquor, is separated from the cellulose. Commercial pulping processes include the soda, the sulfite and the sulfate (also known as kraft) processes. This invention relates specifically to the lignin obtained from softwood pulped using the alkaline kraft or soda processes. In these processes the lignin, dissolved in alkaline black liquor, is combusted in a recovery boiler to produce energy.
Since the kraft process is predominant in the pulp and paper industry, and softwoods are a significant raw material source to this industry, softwood kraft lignin is a major widely produced and available commercial product. To date, the researchers and journals have expressed the opinion that softwood lignin cannot be formed into carbon fiber without the substantial use of additives (solvents) and other enhancements to make the lignin additive admixture meltable and drawable into fibers. To the best of our knowledge, there is no disclosure which describes green lignin fibers melt extruded primarily from softwood lignin or its derivatives, without substantial use of solvents or additives. Accordingly, there is a need to develop a methodology to convert this widely available material into a precursor for carbon fibers.